The present invention is concerned with improving the operation of variable frequency generators (VFG), in particular for controlling the excitation current of a VFG.
A variable frequency generator (VFG) is used in many applications where a power generator is required to generate outputs of different frequencies or voltages to power different loads. For example, modern aircraft power generation systems often use VFGs to provide electrical power to various on-board loads and applications.
Rotation of the main engine shaft is converted, by the VFG, into electrical energy which is then switched to appropriate frequencies as appropriate to power the various on-board applications and loads.
A variable frequency generator includes a permanent magnet generator (PMG), an exciter, and a main generator mounted for rotation on a common shaft. The shaft is driven by a prime mover such as a gas turbine engine.
A generator control unit (GCU) converts alternating current from the PMG to provide DC current to the exciter. Current from the exciter is fed to the main generator, which produces a voltage output.
FIG. 1 shows, as a schematic block diagram, an example of an existing VFG system. Excitation is provided to the VFG by rectifying, with no control—i.e. with a passive rectifier 6—the voltage produced by the permanent magnet generator 1 stage of the VFG, and then a controlled current, created by means of a chopper 7 operating in current-source mode, is injected into the VFG exciter 2 stage. This provides a DC link voltage with a variable magnitude directly dependent on the mechanical input speed to the VFG, from the input shaft 4, varying from a voltage V when the engine is at idle speed, to 2×V when the engine is at maximum (take-off, for aircraft) speed. The relationship between input frequency and DC link voltage is shown in FIG. 3.
Problems can occur in such VFGs leading to overvoltage situations. For example, a failure may occur inside the generator control unit (GCU) at the chopper stage, for example one of the switches may be stuck closed. This can happen, in particular, at high mechanical input speeds, and excitation control can be lost. The current source duty cycle may be brought to its maximum and the VFG output voltage would develop very quickly and become so large as to exceed the maximum voltage acceptable by the electrical loads.
Such overvoltage problems have been previously identified. Known techniques for preventing damage if such failure occurs include inserting an over-voltage protection unit (OPU) in series with the VFG excitation lines. The OPU senses the output voltage and immediately opens the excitation lines whenever an overvoltage occurs.
In systems in which overvoltage protection relies on the GCU monitoring the power supplied by the main generator and then tripping a switch to an open condition once an overvoltage threshold has been reached, further problems have been identified. For example, manufacturing defects or environmental effects may simultaneously disable the GCU and cause abnormally high generator output values. The disabled GCU is then unable to detect or react to these high output values.
US 2013/0003231 provides an alternative overvoltage protection technique involving predicting a primary control current to provide a predicted control current and detecting an overvoltage condition based on a comparison of the predicted control current and the primary control current and interrupting the primary control current based on the comparison.
Such systems, however, still have the problems identified above in that the VFG output overvoltage is dependent on mechanical input speed and can develop to a very high value very quickly, exceeding the maximum acceptable voltage and causing damage if the OPU reaction time is not appropriate.